Wednesday, December 24, 2008

A move to take a new look at atomic energy as a measure to alleviate global warming is gathering momentum around the world. However, concerns about safety, disposal of radioactive waste and nuclear proliferation cannot be dispelled. As a way to solve these problems comprehensively, I wish to call attention to nuclear power generation that uses thorium for fuel.

Thorium is a naturally occurring element that can be used as nuclear fuel. In uranium-fueled nuclear reactors, after absorbing a neutron, part of the uranium with an atomic mass number of 238 turns into plutonium-239, a key fissionable component in nuclear weapons.

With an atomic mass number of 232, thorium hardly breeds plutonium when used as nuclear fuel, reducing fears of nuclear proliferation.

Thorium is distributed extensively, and reserves are rich in such countries as Australia, India and China. It is estimated that thorium reserves are more than four times as abundant as uranium reserves.

In May, then Australian Governor-General Major General Michael Jeffery said that the use of thorium should be considered as a sustainable energy source because it does not produce weapon-grade materials. The use of thorium was also discussed at an international symposium on climate change held in October in Potsdam, Germany.

A statement issued in December 2007 by experts on global warming from Japan, China and India also included the use of thorium.

It is said that molten-salt reactors are most suitable for the use of thorium as a nuclear fuel. Molten salt refers to salts such as sodium chloride that come in liquid form under high temperatures. Energy is produced by dissolving thorium and a small amount of fissile materials in molten salt, which serves as the primary coolant.

Compared with conventional nuclear reactors that use water as a coolant, molten-salt reactors are safer because the pressure of the coolant is low even under high temperatures.

Another advantage is that there is no need to frequently stop operations to exchange fuel because they do not use fuel rods. Nor do they produce transuranium elements--chemical elements with atomic numbers greater than that of uranium at 92. These are the main elements of high-level radioactive waste.

In other words, molten-salt reactors can reduce radioactive waste both in terms of quantity and quality.

Studies on molten-salt reactors advanced in the United States from the 1950s to the 1970s. An experimental reactor was also operated for four years without accidents. But under the Cold War regime, one of the major purposes of nuclear power generation was to obtain uranium and plutonium to build nuclear weapons. Therefore, for political reasons, thorium was not a favored choice.

But now that the Cold War is over and the world is facing global warming, thorium and molten-salt reactors are once again attracting worldwide attention. In the United States in October, Democratic Senator Harry Reid and another senator submitted a bill to set aside $250 million for research and development of thorium-fueled nuclear power generation.

In the Czech Republic, too, construction of a molten-salt reactor is scheduled to start in 2013.

Meanwhile, in Japan, Kazuo Furukawa, a former Tokai University professor, and others have been proposing the use of thorium based on U.S. research results. However, full-scale domestic research and development have made little progress because researchers and budgets have always concentrated on the use of uranium and plutonium.

In the long run, nonthermal energies such as solar power should be promoted as the main source of energy. But during the transition period, nuclear power is needed.

Since thorium is a radioactive material, it obviously must be handled with care. Safety of waste disposal must also be confirmed. Before building a commercial reactor, high temperature containment vessels should be developed and the integrated system should be verified. Still, I believe thorium molten-salt reactors can eventually become a mainstay technology.

Hundreds of thousands of tons of thorium are in stock around the world in the form of residue after extraction of rare earth elements. The volume is large enough to operate molten-salt reactors until the end of the 21st century at the current global output level of 400 million kilowatts.

If Japan takes the lead in establishing thorium reactor technology and provides it to developing countries that are facing tight energy supply amid growing demand, it would also be useful in curbing global warming. Japan should also seriously consider the use of thorium for its own nuclear power generation.

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The author is an assistant professor at Kyoto University specializing in energy system design and assessment.(IHT/Asahi: December 24,2008)